The estrogen receptor (ER) is a member of the nuclear hormone receptor family and functions as a ligand-activated transcription factor involved with the up and down regulation of gene expression. The natural hormone for the estrogen receptor is 17-beta-estradiol (E2) and closely related metabolites. Binding of estradiol to the estrogen receptor causes a dimerization of the receptor and the dimer in turn binds to estrogen response elements (ERE's) on DNA. The ER-DNA complex recruits other transcription factors responsible for the transcription of DNA downstream from the ERE into mRNA, which is eventually translated into protein. Alternatively, the interaction of ER with DNA may be indirect through the intermediacy of other transcription factors, most notably fos and jun. Since the expression of a large number of genes is regulated by the estrogen receptor and since the estrogen receptor is expressed in many cell types, modulation of the estrogen receptor through binding of either natural hormones or synthetic ER ligands can have profound effects on the physiology and pathophysiology of the organism.
A variety of diseases have their etiology and/or pathology mediated by the ER. Collectively these diseases are called estrogen-dependent diseases. Estrogens are critical for sexual development in females. In addition, estrogens play an important role in maintaining bone density, regulation of blood lipid levels, and appear to have neuroprotective effects. Consequently, decreased estrogen production in post-menopausal women is associated with a number of diseases such as osteoporosis, atherosclerosis, depression and cognitive disorders. Conversely, certain types of proliferative diseases such as breast and uterine cancer and endometriosis are stimulated by estrogens and therefore antiestrogens (i.e. estrogen antagonists) have utility in the prevention and treatment of these types of disorders.
There are two different forms of the estrogen receptor, usually referred to as α and β, each encoded by a separate gene (ESR1 and ESR2, respectively). Both ERs are widely expressed in different tissue types, but there are some notable differences in their expression patterns. The ERα is found in endometrium, breast cancer cells, ovarian stroma cells, and the hypothalamus. In males, ERα protein is found in the epithelium of the efferent ducts. The expression of the ERβ protein has been documented in kidney, brain, bone, heart, lungs, intestinal mucosa, prostate, and endothelial cells. Development therefore of selective ligands may therefore preserve the beneficial aspects of estrogen.
Breast cancer is the most common malignancy to affect women and worldwide, the incidence of the disease is increasing. Estrogens, in particular, act as endocrine growth factors for at least one-third of breast cancers, and depriving the tumor of this stimulus is a recognized therapy for advanced disease in premenopausal women, this is achieved by the ablation of ovarian function through surgical, radio therapeutics, or medical means and, in postmenopausal women, by the use of aromatase inhibitors.
An alternative approach to estrogen withdrawal is to antagonize estrogen with antiestrogens. These are drugs that bind to and compete for estrogen receptors (ER) present in estrogen-responsive tissue. Conventional nonsteroidal antiestrogens, such as tamoxifen, compete efficiently for ER binding but their effectiveness is often limited by the partial agonism they display, which results in an incomplete blockade of estrogen-mediated activity. A specific or “pure” antiestrogen with high affinity for ER and without any agonist effect may have advantages over conventional nonsteroidal anti-estrogens in the treatment of estrogen-dependent disease. Fulvestrant is the first of a new class of potent pure anti-estrogens and is completely free of the partial agonist, estrogen-like activity, associated with currently available antiestrogens like tamoxifen.
As such, there is a need for other approaches to antagonize the ER receptor. One approach would be to develop selective ER down regulators or degraders that reduce ER expression at either the transcript or protein level.
Several methods are available for the manipulation of protein levels, including proteolysis targeting chimeric molecules (PROTACs), which contain a ligand that recognizes the target protein linked to a ligand that binds to a specific E3 ubiquitin ligase. It would be desirable to have a small molecule which can simultaneously bind ER and an E3 ubiquitin ligase and which promotes ubiquitination of ER and leads to degradation of ER by the proteasome.
Most small molecule drugs bind enzymes or receptors in tight and well-defined pockets. On the other hand, protein-protein interactions are notoriously difficult to target using small molecules due to their large contact surfaces and the shallow grooves or flat interfaces involved. E3 ubiquitin ligases (of which hundreds are known in humans) confer substrate specificity for ubiquitination, and therefore, are more attractive therapeutic targets than general proteasome inhibitors due to their specificity for certain protein substrates. The development of ligands of E3 ligases has proven challenging, in part due to the fact that they must disrupt protein-protein interactions. However, recent developments have provided specific ligands which bind to these ligases. For example, since the discovery of nutlins, the first small molecule E3 ligase inhibitors, additional compounds have been reported that target E3 ligases but the field remains underdeveloped. For example, since the discovery of Nutlins, the first small molecule E3 ligase mouse double minute 2 homolog (MDM2) inhibitors, additional compounds have been reported that target MDM2 (i.e., human double minute 2 or HDM2) E3 ligases (J. Di, et al. Current Cancer Drug Targets (2011), 11(8), 987-994).
Tumor suppressor gene p53 plays an important role in cell growth arrest and apoptosis in response to DNA damage or stress (A. Vazquez, et al. Nat. Rev. Drug. Dis. (2008), 7, 979-982), and inactivation of p53 has been suggested as one of the major pathway for tumor cell survival (A. J. Levine, et al. Nature (2000), 408, 307-310). In cancer patients, about 50% were found with p53 mutation (M. Hollstein, et al. Science (1991), 233, 49-53), while patients with wild type p53 were often found p53 down regulation by MDM2 through the protein-protein interaction of p53 and MDM2 (P. Chene, et al. Nat. Rev. Cancer (2003), 3, 102-109). Under normal cell condition without oncogenic stress signal, MDM2 keeps p53 at low concentration. In response to DNA damage or cellular stress, p53 level increases, and that also causes increase in MDM2 due to the feedback loop from p53/MDM2 auto regulatory system. In other words, p53 regulates MDM2 at the transcription level, and MDM2 regulates p53 at its activity level (A. J. Levine, et al. Genes Dev. (1993) 7, 1126-1132).
Several mechanisms can explain p53 down regulation by MDM2. First, MDM2 binds to N-terminal domain of p53 and blocks expression of p53-responsive genes (J. Momand, et al. Cell (1992), 69, 1237-1245). Second, MDM2 shuttles p53 from nucleus to cytoplasm to facilitate proteolytic degradation (J. Roth, et al. EMBO J. (1998), 17, 554-564). Lastly, MDM2 carries intrinsic E3 ligase activity of conjugating ubiquitin to p53 for degradation through ubiquitin-dependent 26s proteasome system (UPS) (Y. Haupt, et al. Nature (1997) 387, 296-299). As such, because MDM2 functions as E3 ligase, recruiting MDM2 to a disease causing protein and effectuating its ubiquitination and degradation is an approach of high interest for drug discovery.
One E3 ligase with exciting therapeutic potential is the von Hippel-Lindau (VHL) tumor suppressor, the substrate recognition subunit of the E3 ligase complex VCB, which also consists of elongins B and C, Cul2 and Rbx1. The primary substrate of VHL is Hypoxia Inducible Factor 1α (HIF-1α), a transcription factor that upregulates genes such as the pro-angiogenic growth factor VEGF and the red blood cell inducing cytokine erythropoietin in response to low oxygen levels. The first small molecule ligands of Von Hippel Lindau (VHL) to the substrate recognition subunit of the E3 ligase were generated, and crystal structures were obtained confirming that the compound mimics the binding mode of the transcription factor HIF-1α, the major substrate of VHL.
Cereblon is a protein that in humans is encoded by the CRBN gene. CRBN orthologs are highly conserved from plants to humans, which underscores its physiological importance. Cereblon forms an E3 ubiquitin ligase complex with damaged DNA binding protein 1 (DDB1), Cullin-4A (CUL4A), and regulator of cullins 1 (ROC 1). This complex ubiquitinates a number of other proteins. Through a mechanism which has not been completely elucidated, cereblon ubquitination of target proteins results in increased levels of fibroblast growth factor 8 (FGF8) and fibroblast growth factor 10 (FGF10). FGF8 in turn regulates a number of developmental processes, such as limb and auditory vesicle formation. The net result is that this ubiquitin ligase complex is important for limb outgrowth in embryos. In the absence of cereblon, DDB1 forms a complex with DDB2 that functions as a DNA damage-binding protein.
Thalidomide, which has been approved for the treatment of a number of immunological indications, has also been approved for the treatment of certain neoplastic diseases, including multiple myeloma. In addition to multiple myeloma, thalidomide and several of its analogs are also currently under investigation for use in treating a variety of other types of cancer. While the precise mechanism of thalidomide's anti-tumor activity is still emerging, it is known to inhibit angiogenesis. Recent literature discussing the biology of the imides includes Lu et al. Science 343, 305 (2014) and Kronke et al. Science 343, 301 (2014).
Significantly, thalidomide and its analogs e.g. pomolinamiode and lenalinomide, are known to bind cereblon. These agents bind to cereblon, altering the specificity of the complex to induce the ubiquitination and degradation of Ikaros (IKZF1) and Aiolos (IKZF3), transcription factors essential for multiple myeloma growth. Indeed, higher expression of cereblon has been linked to an increase in efficacy of imide drugs in the treatment of multiple myeloma.
Inhibitors of Apotosis Proteins (IAPs) are a protein family involved in suppressing apoptosis, i.e. cell death. The human IAP family includes 8 members, and numerous other organisms contain IAP homologs. IAPs contain an E3 ligase specific domain and baculoviral IAP repeat (BIR) domains that recognize substrates, and promote their ubiquitination. IAPs promote ubiquitination and can directly bind and inhibit caspases. Caspases are proteases (e.g. caspase-3, caspase-7 and caspace-9) that implement apoptosis. As such, through the binding of caspases, IAPs inhibit cell death. However, pro-apoptotic stimuli can result in the release of mitochondrial proteins DIABLO (also known as second mitrochondria-derived activator of caspases or SMAC) and HTRA2 (also known as Omi). Binding of DIABLO and HTRA2 appears to block IAP activity.
SMAC interacts with essentially all known IAPs including XIAP, c-IAP1, c-IAP2. NIL-IAP. Bruce. and survivin. The first four amino acids (AVPI) of mature SMAC bind to a portion of IAPs, which is believed to be essential for blocking the anti-apoptotic effects of IAPs.
Bifunctional compounds such as those that are described in U.S. Patent Application Publications 2015-0291562 and 2014-0356322 (incorporated herein by reference), function to recruit endogenous proteins to an E3 ubiquitin ligase for degradation. In particular, the publications describe bifunctional or proteolysis targeting chimeric (PROTAC) compounds, which find utility as modulators of targeted ubiquitination of a variety of polypeptides and other proteins (such as estrogen receptor), which are then degraded and/or otherwise inhibited by the bifunctional compounds.
An ongoing need exists in the art for effective treatments for disease associated with overexpression or aggregation of ER. However, non-specific effects, and the inability to target and modulate ER, remain as obstacles to the development of effective treatments. As such, small-molecule therapeutic agents that target ER and that leverage or potentiate cereblon's, MDM2's, and IAPs' substrate specificity would be very useful.